Porous moulds for manufacturing plastic articles



B. LUX

May 9, 1961 Original Filed Feb. 10, 1958 United States Patent C POROUS MOULDS FOR MANUFACTURING PLASTIC ARTICLES Bei mo Lux, Carouge, Geneva, Switzerland, assignor to Pirelli Societa per Azioni, Milan, Italy Original application Feb. 10, 1958, Ser. No. 714,078. Divided and this application Oct. 28, 1959, Ser. No. 849,389

priority, application Italy, Mar. 29, 1957 11 Claims. (Cl. 18-47) This invention pertains to porous moulds for mould ing under pressure articles composed of natural and t synthetic polymers including the vulcanizable ones, and

In order to avoid the occurrence of these bubbles and t to allow the escape of the occluded air or gas, various solutions have been proposed, but none of them have given really satisfactory results. In particular, porous ;1 noulds of metal, intended. to allow the release of the air and gas without any outflowof the materialto be moulded, have been constructed. Unfortunately, the materialcontained in the mould tends to fill the pores in the mould which are consequently occluded to such an extent as to make the mould practically unemployable ina shorttime. It has also been proposed to use moulds in which the surface of the moulding chamber is covered .witha smooth anti-adhesive material. not permeable to gases, but here also the obtained results have not been satisfactory.

The present invention aims at eliminating the above indicated dilficulties by providing porous moulds, having anti-adhesive moulding surfaces of smaller porosity than that of.the mould body. 9 1 Thenio uld constructed in accordance with the present inveutionis characterized in that it comprises a porous metallic body,;of whichthe stratum immediatelyunderlying and including the moulding surface is impregnated with a porous,-;anti-adhesive plastic material which, for a certain limited depth, partially fills the pores in said impregnated stratum. t

The auti-adhesiveplastic materiallu'scd for impregnatihg the porous moulds of my invention can be any syn thetic resin, resistant to temperatures considerably higher {than the normal service temperature of the moulds, and

is inertQsothat it doesnot formchemical bonds or other ibjonds due to surface action; with the rubber being moulded. Byway of'non-limitativefexample, thefollcwing anti-adhesive plastic impregnating -materials, which can be employed for the abovedescribed purposes,

ethylene, and solidsilicon resins.

T1,?"I1htfve discovered that the} porous metallic ,m oulds,

, when impregnated with such a porous anti-adhesive plastic material, have a prolonged permeability to air and gases without any occlusion of the pores of the mould .polytetrafluoroethylene, polytrifiuoromonochlorot t more porous structure.

"ice

by the mouldable rubber. This result is due to the fact that, owingto the presence of the anti-adhesive plastic material, which is also porous, \m'thin the pores of the mould, the porous structure of the treated part of the metallic body of the mould including its moulding surface becomes finer, thus reducing the possibility of the mouldable rubber penetrating within the pores of the mould. u, 2

Moreover, the mouldable rubber is less adherent to the anti-adhesive plastic material than to the metal, and since the surface tension of the rubber is modified by the presence of the anti-adhesive plastic materiahjthe residues of moulded rubber remaining in the pores do not cause their total occlusion, and for this reason, such residues can. be removed during the subsequent moulding operations. 1

My improved mould comprises a porous metallic mould body of aluminum, iron, stainless steel or any, other suitable metallic material, obtained by metallization, sintering, electro-chemical separation, or any other ,known method, which is impregnated, at least in the, stratum immediately underlying and including the, mouldingsurface, with the anti-adhesive plastic material in a state of solution or dispersion, bydipping, spreading, spraying, ortwith other, like known methods. I

This porous metallic mould body is then heated with rapid increase in temperature, in order to evaporate the solvent of the solution, or the dispersing vehicle ofthe dispersion, and reduce the particles of the mass of antiadhesive plastic material remaining iu the pores to an aggregate in theform of a porous structure having intercommunicating minute cells.

The heat treatment is thenvcontinued in order to subject the aggregate to,a sintering process so that, when the treatment is over, the anti-adhesive plastic material contained in the pores of the mould appears as a coherent porous mass having a smaller porosity than that of the mould body.

I have discovered that the efiiciency of the mould can be improved by using the mould in one or more moulding operations on any vulcanizable natural or synthetic rubber, before treating the porou'sflmetallic, body with the anti-adhesive plastic material. p p p The residues of the rubber remaining in the of the metallic body are carbonized during the final heat treatment which is carried out at a high temperature, and in this condition theytoo may be sintered, together with. the particles of the anti-adhesiveyplastic material, thus cooperatingto produce an improved porous structure consisting of the composite particles remaining in the poresof the mould. f

I have further discovered that a salt, for 'insta nce: ammonium nitrate, ammonium. chloride, urea, polyacrylate, or any other. substance, able under heat to release a gas, can be addedto the solution or dispersi on of the anti-adhesive plastic material. In the final heat treatment there is first created inside each pore of,the

mould an aggregate of porous structureconsisting of particles of anti-adhesive plastic material containing. the gas-forming substance. These particles are then sintered and the gas released causes each piarticle tojassume a Itis to be noted that, by virtue of its :metal lic lbodv, the mould is resistant to deformation and has very good characteristics. of thermal. conductivity, ,whereasltth is would not be the case if itwere completely mat er anti-adhesiveplasticmaterial,

For, best efiiciericy ofthemouldthevery orous structure of plastic material stratum does notei rtend to any substantial depth below the moulding surface of the V mould, as it is sutficient to provide an impregnated stratum immediately underlying the moulding surface of the niould. a

In the drawing, I have illustrated an example of my invention as applied to a hollow mould for moulding a rubber syringe, into which the rubber is introduced in the crude fplastic state and compressed air is applied to mould to ease the rubber to expand and assume the herpfthe w Figure 1 of the drawing shows the mould, in half vern s rets: ea r nrai F g r 2 shows' on an enlarged scale a portion of the wall of the mould, as indicated.

In l of the mould consists of, a lower portion 1 and an upper portion 2 which are clamped together position by a clamp' (not shown) which engages annurar'rnn' e; 3 and 4. I i w irsfsfafia 6 ofibody portions 1 and 2 are composediof a porousmetal, 'such as aluminum, iron, stainless steel or other siiitablemetallic material, which is obtained by metallization, sintering, electro-chemical separation or other known method. Wall's 5 and6 are impregnated on their inner moulding surfaces? and 8 with an anti-adhesive plastic materialwhich penetrates into astratum immediately underlying said moulding surfaces to a selected limited depth, as shown at 9 in Figure forminga moulding surface of smaller porosity thari that of the wall portions 5 and 6. The selected limited depth to the anti-adhesive material penetrates into the porous metallic stratum immediat ely underlying the moulding surfaces depends upon the n ture of the particular anti-adhesive plastic material and h r ubber to be moulded, and is determined em pirically; the criterion being that said porous metallic stratum :shall contain a layer of intercommunicating, minutefspace cells sufficient to permit the: escape of occluded air and gas which accumulate in saidstratum during thernoulding operation. i f According to the invention, while the plastic material used to impregnate the metallic porous mould has an antiadhesive nature, said material, on the treated metallic porous' mould', has functions specifically 'ditferent from those of a conventional release agent applied on the nonporous surfa'ceof moulds to originate a very smooth moulding surface, and, consequently, to facilitate the detachment of the moulded article from saidf surface. 'In fact, according to'the invention, the anti-adhesive plastic material serves ,first of vallfto constitute, within each treated metallic pore'of the mould, a sponge having a very, finep o'rous for the escape from the mould 10f, occluded .air and gas, and then cooperates to remove the. possible veryfshort pegs offth'e rnoul ded material, as

for c,e rubber, .Whichare forme 1n consequence. of

the penetration 6i m litiamen within the very 'fine'pores'jof each sponge of anti-adhesive plastic material undenthe .str ong moulding pressure. More precisely said pegs, be' g:in,contact with the anti-adhesive plastic material, do not gamete during the removal of the moulded articles from ther'no'uld, they are easily with ,drawnptogether' Witlisaidarticle, o f the very fine pores into"whi ch thay' we're formed. Therfore no posjsibilitypccurs that said pegsimay be separated by breaking from I said article L du ring itsremoval from the mould may remain an the tine pores of said t s clo ina h'em T e Pe av a m v scop siie and'tliir prsehce in the whple is revealed by the opaque appearance of, the moulded article. in this 1 n ry fill; porous structure of the pg g ofithe dh e plastic materialexisting in 'each tre ed mptalic p e t the mould is avoided," and theair can' 'escape out "through very fine porous structure 'during the moulding operation. L

moulding surface which is impregnated with a porous,

anti-adhesive, sintered, plastic material that. penetrates for a selected limited depth into and partially fills the pores of said stratum immediately adjacent its moulding surface, whereby said surface has a smaller porosity-than that of said body.

2, A porous mould as in claim 1, wherein said porous anti-adhesive plastic material is polytetrafliioroethylene. 3. A porous mould as in claim 1, wherein said porous anti-adhesive plastic material is polytrifluoromonochloro ethylene; i

4. A porous mould as in claim 1, wherein said porous anti-adhesive plastic material is a silicon resin. 5 A porous mould for moulding under pressure rubber articles comprising: a porous metallic body in which at least a stratum immediately underlying and including its moulding surface has its pores partially filled with an anti-adhesive, sintered, plastic material in the form of a microporous structure of such selected porosity that it retains the rubber and facilitates the escape of any gas that may be occluded during the moulding operation.

6. A porous mould for moulding under pressure rubber articles, comprising a porous, metallic bodyhavinga stratum immediately underlying and including its moulding surface impregnated with a particulate anti-adhesive, sintered, plastic material whose particles partially fill'the pores of said stratum wherethey are sintered and form an aggregate of smaller porosity than that of said body. 7. A mould according to claim'6, wherein is added to said material a substance which is heated and forms within said particles a gas'that, upon escaping from said pa-rticles,"improves their porosity.

8. A mould according to claim 6, wherein the said plastic material is applied 'to the moulding'fsurfaceoif the'mould by dipping said surface into a solutiofiof said sion'oflsaid plastic material liquid yehicle, 1 References Cited in the file of this pat ent a UNITED STATES PATENTS 1,592,536 QNeill Julyl3, 1926 1,662,365, Oonstock -M ar. 13,1928 mama. u l sen. 29.19.42

2.682. 0... s ai epti- 2.1.1954 2,779,062 --Stastny Jan."29, -1 9.57 2,811,408 I B raley. -Oct.,2 9', 1957 2,488,446 Swiss Nov. 15, 1959 a FOREIGN PATENTS jifl a 657,085 Great Britain Sept 12, 19151 .sgomt- Great Britain Apr. "953 Great Britain mould by spraying said surface w'tiha 'dispeisedsn'spen- 

1. A POROUS MOULD FOR MOULDING UNDER PRESSURE RUBBER ARTICLES COMPRISING: A POROUS METALLIC BODY HAVING A STRATUM IMMEDIATELY UNDERLYING AND INCLUDING ITS MOULDING SURFACE WHICH IS IMPREGNATED WITH A POROUS, ANTI-ADHESIVE, SINTERED, PLASTIC MATERIAL THAT PENETRATES FOR A SELECTED LIMITED DEPTH INTO AND PARTIALLY FILLS THE PORES OF SAID STRATUM IMMEDIATELY ADJACENT ITS MOULDING SURFACE, WHEREBY SAID SURFACE HAS A SMALLER POROSITY THAN THAT OF SAID BODY. 